Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with magnetic tunnel junction (MTJ) technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Similarly, spin-transfer (spin torque or STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has stimulated considerable interest due to its potential application for spintronic devices such as spin-torque MRAM on a gigabit scale.
Both MRAM and STT-MRAM may have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. The MTJ element is typically formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line at locations where the top electrode crosses over the bottom electrode. A MTJ stack of layers may have a bottom spin valve configuration in which a seed layer, a ferromagnetic “reference” layer, a thin tunnel barrier layer, and a ferromagnetic “free” layer are sequentially formed on a bottom electrode. The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the reference layer. The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons. The magnetic moment of the free layer may change in response to external magnetic fields and it is the relative orientation of the magnetic moments between the free and reference layers that determines the tunneling current and therefore the resistance of the tunneling junction. When a sense current is passed from the top electrode to the bottom electrode in a direction perpendicular to the MTJ layers, a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“0” memory state) and a higher resistance is noted when they are in an anti-parallel state or “1” memory state.
As the size of MRAM cells decreases, the use of external magnetic fields generated by current carrying lines to switch the magnetic moment direction becomes problematic. Spin torque MRAM was developed to provide a robust magnetic switching margin by eliminating the half-select disturb issue. Compared with conventional MRAM, spin-torque MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a CPP configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high. The difference between a spin-torque MRAM and a conventional MRAM is only in the write operation mechanism. The read mechanism is the same.
For MRAM and spin-torque MRAM applications, it is often important to take advantage of PMA films with a large and tunable coercivity field (Hc) and anisotropy field (Hk). For example, PMA films may be used as a pinned layer, free layer, or dipole (offset compensation) layer in a MTJ element or in PMA media used in magnetic sensors, magnetic data storage, and in other spintronic devices. Furthermore, a critical requirement is that Hc, Hk, and other properties such as the magnetoresistive (MR) ratio do not deteriorate during processing at elevated temperatures up to 400° C. or higher. In some applications, it is also necessary to limit the overall thickness of the seed layer and other underlayers below the PMA layer to less than about 100 Angstroms, and use only materials that are compatible with device design and high temperature processing requirements.
Materials with PMA are of particular importance for magnetic and magnetic-optic recording applications. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement and have a low switching current density but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is one of the key challenges for future MRAM applications and spintronic devices.
When the size of a memory cell is reduced, much larger magnetic anisotropy is required because the thermal stability factor is proportional to the volume of the memory cell. Generally, PMA materials have magnetic anisotropy larger than that of conventional in-plane soft magnetic materials such as NiFe or CoFeB. Thus, magnetic devices with PMA are advantageous for achieving a low switching current and high thermal stability.
Several PMA material systems have been proposed and utilized to date such as multilayers of Pt/Fe, Pd/Co, and Ni/Co, and ordered (e.g., L10 structures) as well as unordered alloys but there is still a need for improvement in Hc, Hk, temperature stability, and material compatibility. Among the PMA material systems studied, a Ni/Co multilayer is one of the more promising candidates because of its large potential Hc and Hk, good stability at high anneal temperatures, and potential compatibility with other materials used in magnetic devices. However, Ni/Co multilayers and the like typically require a thick seed layer to induce high PMA. Seed layers thicker than about 100 Angstroms will require thinning a different layer in the spin valve structure to maintain a certain minimum thickness for the MTJ element which can easily lead to performance degradation.
An improved seed layer is still needed that is thin enough to be compatible with spintronic devices, can induce greater PMA in overlying Co/Ni multilayer structures or CoFeB layers, and is compatible with the design and processing requirements of magnetic devices.